The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure. The floor types described below typically have two major common features. They are intended to be installed in a floating manner with a mechanical locking system and the decor is, in most cases, obtained by colour pigments.
The different flooring types described below use two major classes of polymers, thermosetting and thermoplastic polymers, to provide a bonding between particles, mainly wood and mineral fillers. Such fillers are used to improve mechanical properties or just as extenders to decrease the amount of polymers used.
Thermosetting polymers are characterized in that they melt, float and polymerize with increased stiffness when heat and pressure is applied. They are completely cured and cannot be melted again. Thermoplastic particles on the other hand soften and melt when temperature is increased and the stiffness increases when they are cooled down.
Thermoplastic polymers are very moisture stable and may be formed as a product comprising only a specific polymer material. Thermosetting polymers are moisture sensitive and very brittle. They must be combined and reinforced with fibres, generally with wood fibres.
1. Thermosetting Flooring
This flooring category is characterized in that thermosetting resins such as, for example, Melamine Formaldehyde (MF), Urea Formaldehyde (UF) or Phenol Formaldehyde (PF) resins are combined with mainly wood fibres and cured under heat and pressure such that separate core or surface layers may be formed.
a) Laminate Floors
The majority of all laminate floors have a wood design. Such laminated floors have a core of 6-12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer. The decorative and wear properties are generally obtained with two separate layers of melamine formaldehyde impregnated paper (a decorative layer and a wear layer), one above the other. The decorative layer is a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles. The core is generally a high-density fibreboard (HDF) comprising wood fibres and a mix of MF and UF resins. The decorative paper and the overlay paper are directly laminated by a hot-hot production method to the HDF core under high heat and pressure (170° C., 40 bar). Hot-hot pressing generally refers to a pressing method where a press table has essentially the same closing temperature as the opening temperature (e.g., within about 10° C.). Hot-hot pressing is very cost efficient since no cooling of the press table is needed but can only be used in direct lamination of thermosetting based surfaces that cure at high heat and that are laminated to rather thick core materials comprising thermosetting resins that are able to absorb the steam from the moisture of the core and the surface layer that is created at high pressing temperatures above 100° C.
A separate laminate sheet may also be produced in a continuous or discontinuous pressing operation and the laminate sheet is thereafter glued to a core material. This gluing, that may be made in cold condition, allows that separate laminate sheets may be bonded to different core materials, provided that they have similar moisture movements as the surface layer when relative humidity (RH) varies between dry (RH 20%) and wet conditions (RH 90%).
b) Powder-based Floors
Recently new “paper free” floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles hereafter referred to as WFF (Wood Fibre Floor).
The powder mix may comprise aluminium oxide particles, thermosetting resin such as melamine formaldehyde resins and wood fibres. In most applications decorative particles such as, for example, colour pigments are included in the mix. In general, all these materials are applied in dry form as a mixed powder on a wood-based core such as a HDF core and cured in a hot-hot direct lamination step under similar heat and pressure as used in laminate floors.
Digital powder printing has been developed and it is possible to create very advanced wood, stone and tile designs by injecting water based ink drops comprising pigment dispersions into the powder prior to pressing. A recently developed Binder and Powder (BAP) printing method may also be used to provide a digital print on a powder-based surface. Conventional digital printing methods are divided in two separate steps comprising a separate application of a liquid binder, generally referred to as “blank ink” and of dry colorants, generally referred to as “dry ink”. Coloured particles of dry ink are applied on a panel surface. Some particles are bonded by a digitally formed pattern of blank ink applied by a print head. Other non-bonded dry ink particles are removed and the remaining bonded particles form a digital decor. This two-step process may be repeated and several colours of dry ink may be applied such that an advanced multi colour high definition digital print may be formed in a cost efficient way. The major advantage is that rather simple water based liquid substances may be used and the pigment based dry ink colorants are very cost efficient since no pigment dispersions are needed.
2. Thermoplastic Flooring
This flooring category is characterized in that thermoplastic materials such as, for example, polyvinyl chloride (PVC), polypropylene (PP), or polyethylene (PE) are mixed with mineral or wood fibres. The final pressing is made under lower pressure than generally used in, for example, laminate production and with a hot-cold pressing method. The temperature is generally similar, about 140-160° C. Thermoplastic material melts at high temperatures and no curing takes place. The press table or belt must therefore be cooled down below 100° C. such that the thermoplastic layer is converted from a floating paste to a solid layer. Separate thermoplastic layers may be fused or laminated together by heating and cooling under pressure.
c) LVT Floors
Luxury Vinyl Tiles, generally referred to as LVT floorings, are generally constructed as a layered product made of thermoplastic material such as PVC, generally referred to as vinyl, mixed with mineral fillers, colour pigments and plasticizers. The name LVT is somewhat misleading since a major part of LVT floors have a plank size with a wood design. They may also comprise other plastic materials that are not vinyl. LVT has become a generic name for high quality resilient floor that may be installed floating with a locking system.
Thermal moulding is used to form the PVC layers. Co-rotating twin-screw extruders are used for the production of high-quality thermoforming PVC layers with fillers. The screws press and mix the material under heat.
The extruder may be filled with pre-formed small plastic pellets comprising a compound of PVC, pigments, fillers etc. Compounding is a process where mixing of polymers and additives in a molten state under heat and generally also under pressure is used in order to form plastic pellets comprising a homogenous blend of the different raw materials. Co-kneaders, twin-screw extruders and internal mixers are the most common compounders used in the plastic industry.
The compounding process may be avoided and an extruder may also be directly filled with basic plastic particles and separate fillers and other additives that are mixed in a hopper attached to the extruder. This method may be used to save material cost. Advanced twin-screw extruders with optimized screw geometry combined with accurate temperature control are used to obtain high melt homogeneity.
The extruder comprises a flat sheet die that provides a paste with essentially the same width as the finished layer. The die has generally oil temperature-controlled adjustable die lips that provide a wide thickness range.
The melted paste is further processed with 3-roll polishing calenders comprising a fixed central hot roll and two adjustable rolls on each side for calibration. The first roll is hot and the third roll is cold. The melt feed angle, which is determined by the weight and elongation of the melt, can be changed by adjusting the position of the first roll. Horizontal, inclined or vertical roll arrangements may be used.
A wide range of downstream equipment and methods are used to laminate the layers under heat and pressure and to cool the plastic layers to a multi-layer sheet. One simple method is to cut the layers to sheets and to use a multi opening hot-cold press. In continuous production several extruders may be used and the layers are hot when they are laminated together, in most cases with an intermediate layer of glass fibre. Cooling and stabilization of the panel in order to avoid warping takes a lot of space and a continuous production line may be several hundred meters long.
Extruders and calenders may be replaced with a continuous double belt presses comprising heating and cooling under pressure combined with several nip rollers. Plastic compounds in pellet form are scattered on the lower belt and pressed together under heat and pressure. A similar continuous press may also be used to laminate several sheets and foils to a LVT floor panel.
The core is made primarily of several layers that comprise PVC mixed with chalk and/or limestone fillers. The most common core materials comprise a middle part with about 50% limestone and a balancing layer with about 40% limestone. The lower limestone content in the backing is used to balance the high plastic content in the top layer. Some LVT core material may comprise up to 80% limestone fillers. Such core materials with high filler content are very brittle and it is difficult to form a strong locking system.
A 0.1 mm high quality printed decorative PVC foil is applied on the upper side of the core. A transparent wear layer of vinyl with a thickness of 0.1-0.6 mm is generally applied on the decorative foil. The base layer, the decorative foil and the transparent layer are fused or laminated together with heat and pressure in continuous or discontinuous press operations to a 3-6 mm floor panel. A transparent UV curing polyurethane (PU) lacquer is generally applied as a protective layer on the transparent PVC foil. LVT floor are installed floating with a locking system similar to laminate and powder based floors.
The decorative effects are obtained with a decorative foil that is printed separately and fused on the core layers. The foil comprises a single colour that generally is white and covers the dark colour of the core. The foil provides a base colour for the rotogravure printing process where special solvent-based inks comprising pigments are used to create wood, tile and stone designs.
d) WPC Floors
Wood Plastic Composite (WPC) panels comprising thermoplastic material mixed with wood particles have been used in extruded sections and injection mouldings in several industries during many years. Recently WPC flooring panels in plank sizes have been developed for outdoor use mainly as decking. Thermoplastic material such as PVC or PP is mixed with wood in the form of sawdust or wood chips by wood particle weight (wt) of about 50% to 80%. Products intended for exterior use may comprise 50 wt % or less of wood particles. The wood content may exceed 70 wt % in products intended for interior use. The processing is similar to the methods used to produce a LVT floor. A two-step process may be used where wood fibre particles and plastic particles in a first step are compounded together under heat and pressure to plastic pellets and fed into an extruder. Fibres and plastic granules may also be mixed directly during extrusion. Alternatively compounded plastic pellets may be scattered on a belt and pressed to WPC sheets in a continuous double belt press comprises a heating and a cooling section as described above. The decorative properties are mainly obtained by pigments that are included in the wood/plastic mix. WPC panels may also be printed, painted or foiled.
3. Combi-floor
Thermoplastic and thermosetting materials may be combined. Thermoplastic LVT layers may, for example, be glued to a HDF core or a WPC core.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is called “front side or upper side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side or lower side”.
By “up” is meant towards the front side and by “down” towards the rear side. By “vertically” is meant perpendicular to the surface and by “horizontally” parallel to the surface.
By “panel” is meant an essentially flat sheet that comprises a core and a decorative surface formed on the core.
By “board” is meant an essentially flat sheet formed material that is intended to be covered by a surface layer and used as a core in a panel.
Known Technique and Problems Thereof
The general technologies, which have been developed by the industry aiming to improve the properties and cost structures of the above-described floors are described below. The methods may be used partly or completely in various combinations with preferred embodiments of the invention in order to create a board and a panel according to this disclosure of the invention.
The above described floor types have different properties and cost structures. Laminate and WFF floors have superior surface properties related to wear, impact and stain resistance. The hot-hot direct lamination method is very cost efficient. The major disadvantages are that they are hard, moisture sensitive and give a high sound level when people walk on the floor.
LVT floors on the other hand are soft, flexible, moisture proof and silent. However, several properties are inferior to laminate floors. The plastic surface is stain and heat sensitive and the production costs are generally higher than for laminate flooring since hot-cold pressing methods must be used. An LVT floor panel expands 3 mm/m when the temperature increases from 5-65° C.
The major advantage of WPC floors compared to LVT floors is that the wood fibres in the WPC floor counteract thermal expansion. WPC floor are also more rigid and may overcome defects in the subfloor. However, WPC floors are expensive to produce and it is difficult to apply an attractive decor layer that is wear and stain resistance and that has an appropriate moisture movement such that warping may be avoided. WPC floor are harder than LVT floors and the sound level is generally higher.
In general it is easy to form a strong locking system in a HDF core. WPC panels are rather rigid and the locking system may have similar strength or even higher compared to HDF based laminate floors. To form locking systems in a LVT floor is much more complicated due to the fact that the core is thinner and softer. A disadvantage is the fact that the locking strength of a LVT based locking system decreases considerably at increased temperatures when the core becomes softer.
It would be an advantage if a water proof thermoplastic core with sufficient strength to form a mechanical locking system and favourable moisture and thermal properties may be produced in a more cost efficient way and if such a core may be combined with a high quality surface layer comprising thermosetting resins, such as melamine formaldehyde resins. Considerable advantages may be obtained if such surface layer may be directly laminated with a cost efficient hot-hot pressing method to a thermoplastic core.
One problem that is preferably solved is the shrinking of the melamine resin that takes place during pressing when the resin cures and after pressing when the panel is cooled down to room temperature. Such problems have been solved when direct lamination is made against a wood based panel such as HDF and particleboard comprising thermosetting resins. The problems are not solved when lamination is made against a core comprising thermoplastic materials.
Another problem that is preferably solved is the different humidity variations of thermosetting and thermoplastic materials. As an example it may be mentioned that a surface layer comprising wood fibres and thermosetting melamine resins may expand about 0.8% when the relative humidity increases from 20 to 90%. Comparable expansion of a HDF core based on thermosetting melamine/urea binders may be about 0.4% and such a difference is possible to overcome with a strong lamination between the wood fibres in the core and the thermosetting resin in the surface layer. The lower moisture movement of the core will counteract the higher moisture movement of the thermosetting based surface layer and a laminated panel may have a moisture movement which is an average of the two movements, for example, a moisture movement of about 0.6%.
The humidity variations of a WPC panel may be only 0.05% during a time period when a melamine formaldehyde layer expands 0.8%. LVT panels are even more humidity stable and practically no expansion may be measured. Different humidity movements between a humidity sensitive surface layer comprising, for example, melamine laminated to a humidity stable WPC or LVT core comprising thermoplastic material will create large shear forces between the surface layer and the core and this may cause delamination. Furthermore it is also very difficult and in fact practically impossible to laminate a thermosetting layer with a hot-hot direct lamination method to a conventional WPC or LVT layer where all fillers are embedded in a thermoplastic layer. Conventional digital ink jet printing and digital BAP printing as described above are based on the main principle that the ink drops should not float or form clusters when they hit the surface. It is difficult to use digital printing on dense and non-absorbing surfaces such as thermoplastic surfaces. It would be a major advantage if surfaces comprising thermoplastic resins may be formed such that floating of ink drops may be avoided.
WO 96/27721 describes a WPC type of floor panel with a core of wood particles impregnated with a thermoplastic and with a decorative thermosetting laminate as a surface layer glued on top of the core. It is difficult to glue a thermosetting material to a thermoplastic WPC core and the disclosure does not give any description of how the gluing should be made. The materials have different humidity and moisture movements and such panels will warp or the glue layer will break when relative humidity varies between 20 and 90%.
EP 1847385 B1 describes a construction board with a core made of wood chips or wood fibres, which are glue-treated and pressed together, and two cover layers at the upper side and the lower side consisting of scattered WPC material. The core and the WPC cover layers are pressed together under heat such that the thermoplastic plastics materials of the WPC layers are bonded to the wood core. It is mentioned that the surface can be provided with a variety of building board coatings but no specific surface layer is disclosed. The bending strength may be increased and humidity movements may be reduced but a conventional wood based core layer in the middle parts such as particle board, HDF or OSB is not water proof and will swell considerably if exposed to water that may penetrate into the joints between adjacent floor panels.
WO 2008/122668 A1 describes a method of manufacturing laminate floor panels comprising a WPC core, a basic layer of paper or glass fibre melted to the WPC core and a top laminate comprising at least one paper layer impregnated with melamine resin, and being attached to the basic layer through this impregnation material or a plastic layer melted to a glass fibre basic layer. Such a basic layer may provide a lamination between the core and the laminate surface. However the thermosetting laminate layer will shrink and swell more than the WPC core and the panels will warp at low and high relative humidity, the surface or the glue layer may crack or delaminate.
WO 2012/053036 A1 describes a flooring comprising a base material layer of LVT and a single sheet of WPC laminated on top of the LVT. An adhesive layer is provided between the LVT and the WPC layer and a nonwoven cloth is embedded into the adhesive layer.
A disadvantage is that the lower part of the panel comprises a soft LVT layer and this part must be used to form the locking system. The locking strength will be low especially when the temperature increases due to direct sun exposure during the summer months or when floor heating is used.
EP 2402155 A1 shows a floor panel with a WPC layer and a reinforcement layer. The reinforcement layer is intended to minimize dimensional changes of the WPC layer. The reinforcement layer is incorporated within the WPC layer such that portions of the WPC layer extend at both sides of the reinforcement layer. The floor panel includes a high pressure laminate construction comprising at least a printed decorative layer and an impregnated protective overlay pressed together with heat and pressure. Alternatively the WPC layer may be provided with a polymeric film having a decoration pattern, which is printed on the film. The polymeric film may be melted to the WPC layer. Other alternatives are wood veneer layers and vulcanized cellulose layers.
The reinforcement layer may increase the possibilities to bond a thermosetting laminate to a thermoplastic core but it will not eliminate the problems related to different moisture movements of a thermosetting top layer connected to a thermoplastic core.
WO 2009/065769 (Välinge) describes that a panel may be formed by a powder mix of wood fibres and binders. Thermosetting binders may be used in the core and thermoplastic binders in the surface layer or the opposite.
WO 2011/129757 (Välinge) describes a digitally printed surface layer that may comprise a thermoplastic material, for example vinyl, applied in powder form on a substrate. Powder layers may comprise a mix of wood fibres, thermosetting binders in powder form and thermoplastic powder particles. It is also described that a powder based surface layer comprising thermoplastic particles may be applied on a core, preferably of plastic or wood, which may comprise one or several layers with different density. A LVT floor panel can be produced with a powder based and digitally printed surface layer.
US 2003/0008130 describes a water resistant particleboard comprising wood particles, thermosetting resins and resin particles of a thermoplastic resin. The particleboard comprises thermoplastic sheet material at the upper and lower side. The wood and thermoplastic particles have an average particle diameter of 1-2 mm. The average particle size of the outer layers is smaller than the particle size of the middle layer.
The three applications above do not provide solutions to bonding and moisture movements of a surface layer comprising thermosetting binders laminated to a core comprising thermoplastic material.
As a summary it may be mentioned that none of the above described methods may be used to obtain a considerable improvement of the surface properties, stability and cost structures of a thermoplastic based floor intended to be installed floating with a strong locking system.
Objects and Summary
A general objective of at least certain embodiments of the invention is to provide a thermoplastic based building panel that has better surface properties and more favourable cost structure that known building panel types.
A first specific objective of at least certain embodiments of the invention is to provide a panel, preferably a building panel, with a core comprising thermoplastic material, which gives the panel improved water resistant properties, and a surface comprising thermosetting resins that give the panel improved surface properties. The core and the surface should be such that a strong bonding may be obtained with heat and pressure and such that humidity movements of the materials in the core and in the surface are adapted to reduce warping and large shearing forces.
A second specific objective of at least certain embodiments of the invention is to form a board material that may be used as a core in such panels.
A third specific objective of at least certain embodiments of the invention is to adapt the core such that a strong locking system may be formed in such floor panels.
Embodiments of the invention are based on an understanding that conventional wood plastic composite materials such as WPC are mainly designed for construction purposes where high loads must be handled for a long time in outdoors environment. Performance optimising has been directed towards high bonding strengths between the wood fibres and plastic material, UV stability, bending strengths, deformation under load and similar. Several additives are included such as coupling agents that increase the bonding between the wood particles and the thermoplastic material and UV stabilizers that counteract colour changes. A mix of wood and thermoplastic particles is melted under pressure to form compounds of wood/plastic pellets that provide a homogeneous high quality panel after pressing or extrusion.
Such optimising is not suitable for floating floor applications where the panel is laying on a supporting sub floor in indoor environment and is covered by a protective surface layer. In fact it provides an unnecessarily high cost structure and the material properties can cause major problems related to the possibility of applying a surface layer comprising thermosetting resins especially if direct lamination is used based on a hot-hot pressing step. No attempts have been made to form a board that is adapted to provide a strong locking system.
Embodiments of the invention are based on a second understanding that the material properties of the core and preferably also of the surface layer must be changed compared to conventional technology in order to form a panel that has all the favourable core properties of thermoplastic based floor types but also the favourable surface properties of laminate and wood powder based floors where the upper layer comprises a high content of melamine formaldehyde resins. In general terms it means that a core comprising thermoplastic material should be adjusted such that it performs more in a way similar to a layer formed by thermosetting resins. It is preferred that the surface layer comprising thermosetting resins is adjusted such that is behaves more as a thermoplastic material.
According to a first aspect of the invention a method to produce a building panel is provided. The method comprises the steps of:                providing a core by forming a mat shaped layer comprising a dry blend of wood particles in powder form and thermoplastic particles in powder form, heating and cooling the mat shaped layer under pressure such that a sheet is formed after pressing;        applying a surface layer comprising a powder mix comprising wood particles and thermosetting resin on the core, and;        laminating the powder based surface layer in a hot-hot pressing operation to the core and thereby forming a building panel, wherein the wood particles of the surface layer and of the core are bonded to each other by cured thermosetting resin.        
The sheet formed after pressing may be flat.
The surface layer may further comprise a thermoplastic material. The thermoplastic material may be located between the bonded wood particles. The thermoplastic material may comprise thermoplastic particles. In particular, the thermoplastic material may comprise melted thermoplastic particles. The thermoplastic particles may be comprised in the powder mix.
Forming the board from a dry blend and mixing thermosetting particles into the surface layer will increase the moisture movement of the core, decrease the moisture movement of the surface layer and improve the bonding properties such that a direct lamination in a hot-hot pressing operation may be made of the surface layer to the core.
The thermosetting resin may be an amino resin such as melamine formaldehyde resin, urea formaldehyde resin, a phenol formaldehyde resin, or a combination thereof.
The thermoplastic particles of the dry blend may comprise polyvinyl chloride (PVC), polypropylene (PP) or polyethylene (PE).
The thermoplastic particles of the dry blend may have an average particle size of about 0.3 mm or less.
The surface layer may be applied as two layers arranged above each other and an upper layer of the two layers may comprise bleached fibres and thermosetting particles. The bleached fibres may be bleached cellulose fibres.
The wood particles of the core may prior to application be dried to a moisture content of about 2% or less.
The wood particles of the core may prior to application be sieved to an average size of 1.0 mm or less.
The content of the wood particles in the core may be in the range of about 30-80 wt %.
The core may be formed with a pressure which is lower than 20 bar.
The thermoplastic material may be comprised in the powder mix of the surface layer and may comprise thermoplastic particles. Moreover, the thermoplastic particles may have an average size of about 0.2 mm or less.
The wood particles of the surface layer may be smaller than the wood particles of the core.
The core may comprise an intermediate layer and an upper layer. The intermediate layer may comprise a higher amount of thermoplastic material than the upper layer.
The intermediate layer may further comprise limestone particles.
The surface layer may comprise a transparent wear layer comprising thermosetting resins, wear resistant particles, such as aluminium oxide particles, and cellulose fibres, preferably bleached cellulose fibres.
The wood fibers of the core and/or the surface layer may be cellulose fibres, such as bleached cellulose fibres.
The thermosetting resin of the transparent wear layer may be an amino resin such as a melamine formaldehyde resin, a urea formaldehyde resin, a phenol formaldehyde resin, or a combination thereof.
According to a second aspect of the invention a building panel comprising a core and a surface layer is provided. The core comprises a mix of thermoplastic material and wood particles. The surface layer comprises a composite material comprising wood particles bonded to each other by a cured thermosetting resin. The surface layer is bonded to the core by the cured thermosetting resin.
The surface layer may comprise a composite material comprising individual thermoplastic particles spaced from each other and surrounded by wood particles bonded to each other by a cured thermosetting resin.
The surface layer may be bonded to the core by fused thermoplastic particles and cured thermosetting resins.
The thermosetting resin may be an amino resin such as a melamine formaldehyde resin, a urea formaldehyde resin, a phenol formaldehyde resin, or a combination thereof.
The thermoplastic material of the core may comprise PVC, PP or PE.
The surface layer may comprise thermoplastic particles with a size of about 0.2 mm or less.
The surface layer may comprise wood particles with an average size that is smaller than the average size of the wood particles of the core.
The building panel may be a floor panel.
One pair of opposite edges may be formed with a mechanical locking system comprising a protruding strip. Moreover, the protruding strip may comprise wood particles and thermoplastic material.
According to a third aspect of the invention a building panel having a core, a surface layer and a two opposite edges comprising a mechanical locking system is provided. The building panel comprises a tongue and groove for vertical locking and a protruding strip with a locking element in one edge cooperating with a locking groove in another edge for horizontal locking. The core is formed with three layers of materials. Each layer comprises thermoplastic materials and fillers. An upper and a lower layer comprise fillers of wood particles and an intermediate layer comprises fillers of mineral particles.
The locking element may be formed in the lower layer. Moreover, the protruding strip and the locking element may be formed in the lower layer. Additionally, the protruding strip, the locking element and the locking groove may be formed in the lower layer.
The locking element and the locking groove may be formed in the lower layer.
The building panel may be a floor panel.
The building panel may comprise a surface layer comprising wood particles and thermosetting resins.
The principles of embodiments of the invention may be used to form a board that may be used as a core in building panels, especially floor panels that comprise a surface layer with thermosetting resins that is directly laminated to the core. The board surface is adapted to such lamination since a considerable amount of free wood fibres are exposed at the surface and may be bonded with cured thermoplastic resins from the surface layer.
According to a fourth aspect of the invention a board is provided. The board comprises an upper surface, a lower surface, and an intermediate portion located between the upper surface and the lower surface. The intermediate portion comprises wood particles embedded in a thermoplastic material. The upper surface comprises essentially raw wood particles adapted to bond to a thermosetting resin.
At the upper surface, the essentially raw wood particles may protrude beyond the upper surface.
The thermoplastic material may comprise PVC, PP or PE.
The average size of the wood particles in the upper surface may be about 0.1-1.0 mm.
At the lower surface, wood particles may protrude beyond the lower surface.